Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. These integrated circuits typically combine many transistors on a single crystal silicon chip to perform complex functions and store data. Increasing the functionality or capacity of a given circuit generally requires a corresponding increase in the number of transistors which must be integrated on a single chip. This increase is often accomplished by decreasing the size of transistors and other devices contained on a chip. As the size and distance between individual devices on a chip diminish, the technical challenges related to design and production of the circuit multiply.
One of the challenges in scaling VLSI (very large scale integration) circuit designs is coping with decreased mask alignment tolerance. Devices on a typical circuit are connected by two or more patterned layers of conductors, each separated from the others by insulating layers, or interlayer dielectrics. Vias are holes formed in an interlayer dielectric to allow electrical connections to the conductors or devices below the insulator. Masks are used to create a pattern for each layer of conductors and to create a pattern for the vias passing through the interlayer dielectric. If the masks are not aligned to each other, the vias may only partially expose the desired conductor or device. If the misalignment is severe, some connections will be shorted to other conductors or left open, and the circuit will not operate. Conventionally, this problem is solved by designing tolerance into the via connections, which requires more space on the chip, or by use of more elaborate mask alignment procedures and equipment.
A second challenge in VLSI design is the control of capacitance between adjacent conductors. In one aspect, capacitance is a measure of the ability of a signal travelling along one conductor to produce an undesirable signal (crosstalk) in a neighboring conductor. This may result in a limitation on the ultimate speed of the device, among other things. Generally, a capacitance effect exists between any two conducting regions separated by an insulator. The capacitance is directly related to the common area of the conducting regions, as well as to the dielectric constant of the insulator between them. The capacitance also increases as the distance between the two conducting regions is decreased.
Conventional semiconductor fabrication uses silicon dioxide or similar insulating materials as both a gap filler between adjacent conductors at the same level and as an interlayer insulator. Silicon dioxide has a dielectric constant of about 3.9. This constant is based on a scale where 1.0 represents the dielectric constant of a vacuum. Various materials exhibit dielectric constants from very near 1.0 to values in the hundreds. As used herein, the term low-k will refer to a material with a dielectric constant less than 3.5.
There exists a need for processes that can produce semiconductor devices with a satisfactory low-k material as an insulator between adjacent conductors. Unfortunately, many other factors besides dielectric constant have affected the maturation of such a process. For example, structural rigidity, method of application, temperature constraints, response to etchants, heat transfer, contamination of surrounding structure, and shrinkage are some of the factors which must also be considered when selecting new dielectric materials or processes. Several techniques have been attempted in an effort to solve this problem. One proposed technique uses a material containing hollow spheres, the air in the spheres serving to decrease the dielectric constant of the material. One of the major problems with this technique is the manufacture of such spheres at a size small enough to easily flow between conductors spaced at a fraction of a micron. Another proposed technique uses various porous materials, including porous oxides. The etching of such materials is very hard to control because of their inherent permeability. Furthermore, they tend to possess poor structural characteristics. Dielectrics formed from polymers or organics have several drawbacks, including low temperature constraints which can limit further processing steps, lack of structural rigidity, poor heat transfer, and etching problems. In particular, several candidates which appear promising have been affected by uncontrollability of the etching process.